A particulate therapeutic agent may be carried by a carrier gas to a patient's lungs to treat the patient. For example, the carrier gas may be oxygen, and the therapeutic agent may be a liquid particulate or solid particulate. Such a mixture may be an aerosol. Such a mixture may be formed by nebulizing a liquid into the carrier gas. This method of administration may be advantageous when the therapeutic agent targets a specific site of action that is readily accessed by inspiration, or when pulmonary uptake of a therapeutic agent from the systemic circulation is suboptimal, or when systemic blood levels of a therapeutic agent are to be minimized. Examples of therapeutic agents that may be administered in this manner include bronchodilators, steroids, antibiotics, mucolytics, DNA lytic enzymes and genetic vectors for the treatment of lung disease. For example, antifungal and antiviral agents have been administered as aerosols. Further, chemotherapeutic agents for pulmonary malignancy and a host of pulmonary therapies having potential systemic toxicity or undesirable systemic side effects have been nebulized. Additional therapies may include surfactant administration, perfluorocarbon delivery and liposomal drug administration.
The respiratory cycle of a human being is comprised of inspiration and expiration. Inspiration accounts for roughly ¼ of the respiratory cycle and expiration accounts for roughly ¾ of the respiratory cycle. If a therapeutic agent is continuously moved past a patient's mouth, roughly ¾ of the therapeutic agent may not be inspired, and therefore may be wasted. Furthermore, the therapeutic agent that is inspired may be only partially taken up by the lung. For certain therapeutic agents, such as surfactant, more than 90% of inspired therapeutic agent may be subsequently expired (rather than retained in the lung) and thus lost in the expired air. This effect may be greatest when the therapeutic mixture has particulates that are especially small, as may be the case when the therapeutic agent has low surface tension (like surfactant and perfluorocarbon) or is a high density liquid (like perfluorocarbons).
One solution to these inefficiencies of dosing is to rebreathe expired gas, thus renewing the opportunity to take up the therapeutic agent rather than lose it to the environment after expiration. If a “single puff” of the therapeutic agent could be repeatedly rebreathed, ultimate uptake could approach 100%. The same would be true for a “puff” followed by prolonged breath-holding. Repeated rebreathing of the same “puff”, or prolonged breath holding may give rise to hypoxia, hypercarbia, discomfort, or may prolong the time required to deliver a requisite dose of the therapeutic agent. Further, a high level of patient cooperation may be required. These problems complicate, if not prevent, use of these approaches to administer a therapeutic agent.
One solution to these problems involves intermittent dosing, in which the therapeutic agent is provided only during inspiration. This solution offers a significant gain in efficiency over continuous dosing, but is still limited in its usefulness by loss of unabsorbed therapeutic agent to the environment during expiration. This solution may be combined with breath-holding to further reduce expiratory losses, but the breath-holding approach may limit the rate of administration of the therapeutic agent and may require a high level of patient cooperation.